Motor control circuit

ABSTRACT

Described is a control circuit or energizing circuit for a series field  mr. To limit the surge of inrush startup current to no greater than the steady-state running current, a series string of three thermistors is inserted in series with the motor. The thermisters have negative temperature coefficient, and have nominally equal ratings and values. The circuit has application in motor/blower systems.

The invention described herein may be manufactured, used and licensed byor for the Government for Governmental purposes without the payment tome of any royalties thereon.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to motor control circuits, which could also beconsidered as motor energization circuits. More particularly, theinvention is directed to, and has for an object provision of, a circuiteffective to insure that the controlled motor shall draw essentially thesame magnitude motor-energizing current when it starts up as it drawswhile in steady-state running condition, and no greater than the runningcurrent.

The invention has application in a motor/blower system usingseries-field-type (or simply "series") universal motors. Such a motorcharacteristically draws a very high starting current (in a conventionalenergization circuit), which may be up to ten times its running current.

In a working embodiment of the invention an alternating currentuniversal series motor was employed. However, the invention could beutilized with direct current series field type motors, and inapplications other than motor/blower systems, i.e. in other applicationswhere limitation of starting current is desirable.

In arriving at the present invention, several approaches wereconsidered, but had to be rejected for reasons discussed below. Solidstate silicon-controlled-rectifier control or triac control had to beruled out because of excessive electromagnetic interference generated byswitching transients.

Another approach was to place a negative temperature thermistor inseries with one of the motor leads. The thermistor chosen was capable ofhandling the currents (5 amp per motor) and limiting the startingcurrent to the maximum running current. However, the time constant ofthe thermistor did not sufficiently match the startup time (or "startingtime constant") of the motor and its load, here the blower. The lack ofsuccess in this approach led to the successful approach of theinvention.

SUMMARY OF THE INVENTION

In accordance with the invention, the single thermistor mentioned aboveis replaced by a circuit arrangement of plural thermistors having acomposite time constant which matches that of the motor/loadcombination.

More specifically, selected were three thermistors of lower resistancevalue, and connected in series with each other and with the motor lead.The three thermistors had identical nominal ratings as to initial("cold") and final ("hot") resistance values, and current ratings. Thismatched the motor startup characteristics and effectively limited theinrush current.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the followup, more detailedspecification of which the appended Claims and Abstract form parts, whenconsidered together with the accompanying drawing, in which:

FIG. 1 is a schematic drawing of a motor control circuit in accordancewith a preferred embodiment of the invention; and

FIG. 2 are graphs of current-magnitude vs. time for a conventional motorenergization circuit and that of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, shown therein is a motor control circuit orenergization circuit in accordance with a preferred embodiment of theinvention. The motor M comprises armature A, and in series circuitrelation therewith, two series field coils FC1 and FC2 which arecircuit-wise located on either side of the armature A, and from whichoutgo the motor leads 1 and 2 respectively. Connected in series withlead 1 is a string of series connected thermistors referred to as agroup as T, and individually and in left-to-right order (leading tolead 1) as T1, T2, T3. The thermistor T1 is connectible to one side orline of the single-phase alternating current (AC) energizing mains,whereas the lead 2 is connectible to the other side or line via on/offswitch S. Motor current is indicated by I.

In the preferred embodiment which was actually constructed andsuccessfully tested, each thermistor T had a nominal resistance value of30 ohms at a temperature of 25° C., and of 0.1 ohms at 150° C.

FIG. 2 illustrates the effects on input current or energizing current asa result of the insertion of the thermistors T in series with the motor.In FIG. 2, the graph G0 is a plot of input or energizing current vs.time for the motor of the motor/blower system here under contemplation,utilizing conventional input-connections to the motor, i.e. lackingsurge protection; G1 is the plot for the same motor and samemotor/blower system but employing surge protection, i.e. the inventivecontrol circuit of FIG. 1.

As may be seen from the graph G0, the conventional approach produces aninitial inrush current ten times the magnitude of the running current.In contrast, graph G1 indicates that the startup current at no timeexceeds the running current. The insertion of the thermistors doesproduce a reduction in operating current, even under runningsteady-state-condtions, vis-a-vis the running current of theconventional energizing circuit, but the reduction is less than 5%. Intesting the actually constructed embodiment of the invention, the lessthan 5% reduction did not adversely affect motor/blower operation.

The described invention admits of several modifications. For example, itis readily adaptable to direct current motor systems in place ofalternating current motor systems. Also, the invention is applicable toother motor-field circuit configurations, so long as one of the fieldcoils is in series with the armature. Furthermore, other pluralthermistor circuit combinations, besides the series of three, mayprovide the correct time constant for matching the motor current-timecharacteristics.

I claim:
 1. A motor control circuit for a series field type motorcomprising the motor and its series field, and connected in seriestherewith, a plurality of series-connected thermistors.
 2. A motorcontrol circuit according to claim 1, wherein the thermistors havenegative temperature co-efficients.
 3. A motor control circuit accordingto claim 2, wherein the thermistors have substantially the same value.4. A motor control circuit according to claim 3, wherein the number ofseries connected thermistors is three.
 5. A motor control circuitaccording to claim 4, wherein each thermistor has a nominal value of 30ohms at 25° C.
 6. A motor control circuit according to claim 1, whereinthe motor is an alternating current series motor, and the circuit isenergized by an alternating current source.
 7. A motor control circuitaccording to claim 1, wherein the motor is incorporated in amotor/blower system.